(1) Field of the Invention
This invention relates to a method for modifying the surface properties of polymer substrates. More particularly, the invention relates to the above-mentioned method which can be carried out rapidly and effected evenly to the surfaces of polymer substrates by subjecting the polymer substrate surface to electric discharge treatment and then photo-grafting radically polymerizable compounds to the polymer substrate surfaces.
(2) Description of the Prior Art
It has been important for widening the uses of polymers and heighten the utility values of polymers to vary only the surface properties of the polyemrs without changing their own mechanical, optical and other properties. For example, since the surface of polyolefin substrate has a large non-polar property, there are several problems in its adhesiveness, printability, adaptability for coating and so forth. In order to solve these problems, there have been tried various kinds of surface treatment methods such as corona discharge treatment, oxidation treatment, frame treatment, surface grafting treatment using radioactive rays and surface coating. These methods are, however, not always satisfactory in view of their effects and expense. In the methods of the corona discharge treatment, oxidation treatment, flame treatment and radiation treatment, the polymer substrates themselves are subject to the influences of the treatments, for example, the optical properties and the mechanical properties of the substrates are adversely affected. Furthermore, in the corona discharge treatment, the effect of the treatment is small and the uniformity and durability of the treatment are poor. In the surface grafting treatment using radioactive rays, even though products of high quality can be obtained, the operation of the treatment is complicated and the treatment is expensive. Further, the surface coating method is disadvantageous not only in that the adhesiveness between the coated layer and the surface of polymer substrate is not good but also in that the optical property and the mechanical property which are inherent to the polymer substrate are lost because of the double layer structure of the coated layer and the polymer substrate having different properties.
In order to eliminate the above-mentioned disadvantages, the present inventors have already proposed new treatment methods for modifying the surfaces of polymer substrates by photochemical graft reaction, for example, in Japanese Published Unexamined Patent Application (Kokai Tokkyo Koho) Nos. Sho. 52-148564 (1977) and Sho. 52-152972 (1977). In these methods, the surface properties of polymer substrates are modified by causing the photochemical graft reaction of radically polymerizable compounds to the surface of polymer substrate by applying actinic rays. However, the following problems are left unsolved. That is, in the first place, the treatment must be done rapidly in order to enhance the productivity. Especially, as described later, when the active sites which are necessary for the graft reaction are few on the surface of polymer substrates, it is necessary to irradiate with them actinic rays for a long time. Secondly, since the majority of photopolymerizable compositions containing radically polymerizable compounds have polarity (solubility parameters are large), the wetting property of the compositions to the non-polar polymer substrates (solubility parameters are small) or the polymer substrates of high crystallizing property, are poor, so that the rate of photochemical graft reaction becomes low and the treatment is liable to become uneven due to partial cissing. Furthermore, it is necessary to use thickening agents, surface active agents or solvents in order to prevent cissing.